Memory element and method of making protective coating therefor

ABSTRACT

AN IMPROVED PROTECTIVE COATING FOR A FILM OF FERROMAGNETIC MATERIAL DEPOSITED ON A CONDUCTIVE CYLINDRICAL SUBSTRATE, WHEREIN THE COATING COMPRISES ON ALLOY OF NICKEL AND TIN CONTAINING BETWEEN 29% AND 37% NICKEL.

Aug. 8, 1972 MEMORY ELEMENT COATING THEREFOR Filed June 30, 1970 4b (mA/ o 1" I I m I I I 0 0 I a o l co I, d,!

u in 1" I I! U g l I" I 9 p z a) m 's :2 5% N m INVENTORS Vcbr M %NLA EMMA Hmk Bud 2m; ga n United States Patent 3,682,604 MEMORY ELEMENT AND METHOD OF MAKING PROTECTIVE COATING THEREFOR Ren Fernand Victor Girard and Hubert Lucien LOlJlS Baton, Grenoble, France, assignors to Societe Industrielle Bull-General Electric (Societe Anonyme), Paris,

France Filed June 30, 1970, Ser. No. 51,194 Claims priority, applies??? France, July 2, 1969,

Int. Cl. B32b 15/02; C23b 5/58 U.S. Cl. 29-193 3 Claims ABSTRACT OF THE DISCLOSURE An improved protective coating for a film of ferromagnetic material deposited on a conductive cylindrical substrate, wherein the coating comprises an alloy of nickel and tin containing between 29% and 37% nickel.

BACKGROUND OF THE INVENTION This invention relates to improved protective coatings for thin magnetic films.

The ability to utilize thin magnetic films for fast memories, of reduced size and of large storage capacity, results from the fact that such films are capable of assuming different stable magnetic states and of transferring from one state to the other in a very short time, of the order of a few nanoseconds, by undergoing a reversal of the magnetization. These films are usually obtained by depositing a ferromagnetic material on a substrate by electrolytic means or by evaporation under vacuum, the deposition occurring in the presence of an orienting magnetic field in order to provide a uniaxial anisotropy of magnetization; i.e., a direction, termed the easy axis, along which, when the orienting field is removed, the magnetization of the film remains preferentially oriented. The ferromagnetic materials most commonly employed for these films consist generally either of binary alloys of iron and nickel or of nickel and cobalt, or of ternary alloys of iron, nickel and cobalt. These alloys may also contain, on occasion, other materials such as aluminium, silicon, manganese, titanium, etc.

After the magnetic material has been deposited on its substrate, it is necessary to protect it to conserve its magnetic properties, not only against the oxidizing action of the air, but against the corrosive action of humidity and of various acids which are found, even though in very small proportions, in the atmosphere. For providing this protection, it was first proposed to cover the magnetic material with a protective layer of insulating material. For this purpose, various insulating materials have been utilized, among which are, for example, polyethylene terephthalate, varnish, and synthetic resins. However, these materials have the disadvantage of not being thermally stable. Therefore, there is the danger that these materials no longer provide effective protection for the ferromagnetic material when, after accidental heating, they undergo plastic deformation or volatilize more or less rapidly. Furthermore, not only do these insulating materials possess only a relatively low mechanical resistance, but they ultimately impair, sooner or later, the magnetic properties of the ferromagnetic film on which they are deposited, because of chemical interactions which take place between these materials and the film. Consequently, to avoid the ferromagnetic material not being provided suflicient protection after a certain time because of the progressive deterioration of the protective coating under the action of heat, even the relatively low heat which is always produced during normal operation of the memory, it has been found preferable for assuring the protection of the film to utilize certain metals which present relatively high melting temperatures and practically no reaction to atmospheric agents. Such metals include, for example, chromium, gold, platinum and rhodium.

However, these metals are particularly costly and, with the exception of rhodium, not only do they not possess all of the hardness desired, but they present the serious disadvantages of diffusing into the interior of the material constituting the magnetic fihn, thereby forcing a distinct diminution of the magnetic properties of the film. Moreover, even the use of rhodium is not fully satisfactory. Rhodium, being a relatively good conductor of electricty, supports the flow of eddy currents when, under the action of magnetic fields generated by current pulses in control conductor pairs of the film, the magnetization in the film turns to align itself in the direction of the resultant applied magnetic field. Again when the applied magnetic field is terminated and the magnetization of the film returns to its alignment in the direction of the easy axis, the rhodium layer supports eddy currents. This eddy current eifect increases in significance with the thickness of the protective conductive layer, and, therefore, is an important factor, because to assure efiective protection it is necessary to cover the film with a substantially thick layer of rhodium, of the order of a few microns. As a consequence, not only are the readout signals induced by the rotation of the magnetization of the film distorted but, in addition, the amplitude of these signals is substantially reduced, whereby costl amplifiers must be added to permit effective triggering of the switching circuits to which these readout signals are applied.

Therefore, it is the object of the present invention to remedy these disadvantages and provide a protective coating for a magnetic film, a coating which resists the action of heat and of the corrosive atmospheric agents, which presents a substantial hardness, which is non-magnetic, which does not support the flow of eddy currents, and which does not alter the magnetic properties of the film on which it is deposited.

SUMMARY OF THE INVENTION tin, containing a proportion of nickel between about 29% and 37%.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described with reference to the accompanying drawing, wherein:

The single figure shows curves representing the variation of the proportion of nickel in the nickel-tin alloy deposit, as a function of the current density and temperature of the electrolytic bath employed for obtarmng this alloy.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the example to be described, the layer of nickel and tin alloy which is provided, in accordance with in the invention, for protecting magnetic film is applied on a film for which the substrate constitutes a cylindrical conductive wire of small diameter. However, this alloy of nickel and tin can also be applied on a magnetic film deposited on any form of substrate, for example a plane substrate. This alloy of nickel and tin on a magnetic film is deposited by electrolytic means, which permits for obtaining a protective coating uniformly distributed over all the surface of the magnetic film.

Normally, when a magnetic film is coated with a layer of protective material, it is necessary that the layer have a thickness sulficient to assure adequate protection of the film. However, where the material forming the protective layer is constituted of a metal or a good conductive alloy, this requirement is more difiicult to realize since, above a certain thickness of the layer, particularly troublesome eddy currents begin to appear at the time of switching of the film, causing heating which ultimately alters the magnetic properties of the film and substantially reduces the amplitude of the readout signals. Moreover, it is known that the power dissipated by eddy currents is proportional to the square of the thickness of the protective layer and inversely proportional to the electrical resistivity of the material of which the layer is constituted.

However, by utilizing an alloy of nickel and tin containing a proportion of nickel between substantially 20% and 37% for coating a magnetic film, these disadvantages are found to be virtually eliminated, because the resistivity of such alloy is relatively high; i.e., of the order of to 100 times that of copper. The eddy current only commence to become undesirable when the thickness of the protective layer attains a relatively large value, of the order of several microns. However, it has been found that for a thickness equal or greater than 5,000 angstroms, the magnetic film is effectively protected. Therefore, when there is utilized for covering magnetic film, the above-mentioned nickel-tin alloy with a thickness of between 5,000 angstroms and a few microns, the film is perfectly protected, whereas the eddy currents are particularly eliminated.

Furthermore, this alloy of nickel and tin which contains 29% to 37% of nickel, has the advantage of conserving the same physical state up to high temperatures, of the order of 400 C. Because of such good thermal stability, a magnetic film which is covered with a layer of this alloy remains effectively protected, even if, in the course of normal operation, considerable inadvertent heating is produced.

In addition, the capability of soldering to this alloy is remarkable, which is an advantage at the time of the soldering of the conductors which control the memory and receiving the readout signals. Finally, this nickel-tin alloy presents a very great hardness which permits it to strongly resist penetration and friction. The hardness defined here is the Vickers hardness, which is measured by the progressive driving into the material, under a load P expressed in kilograms, of a diamond indenter, of pyramidal form perpendicular to a square base. The value H of this hardness is given by the relation:

in which d designates the length, expressed in millimeters of the diagonal of the pyramidal indentation. In the case of the nickel-tin alloy considered, it is found that this Vickers hardness, which varies with the content of nickel in the ,alloy, remains between 400 and 700.

In the embodiment described, the substrate of the magnetic film is constituted of a cylindrical wire of beryllium-copper, coated with a layer of copper of a few microns thickness. This wire, microns in diameter and of great length, traverses from one end to the other an electrolysis tank containing a bath adapted for depositing on the substrate a thin film of ferromagnetic material. The bath consists, for example, of an aqueous solution of salts of iron and nickel, providing for depositing on the wire an alloy of iron and nickel containing about 18% iron. The wire is moved through the tank by a suitable driving arrangement which provides for pulling the wire at a constant velocity. In the example described, this velocity is of the order of 10 meters per hour.v

After having passed through such tank, the wire covered with is magnetic film is subjected to a rinsing, and then enters another electrolysis tank containing a bath of which the composition will be indicated hereinafter, this latter bath being intended for forming on the wire a protective coating of nickel-tin alloy containing 29% to 37% of nickel. In the course of its passage through this latter tank, the wire is completely immersed in the bath. To deposit the protective layer of nickel-tin, the wire is utilized as the cathode and is surrounded by an anode of nickel of appropriate form, which provides for assuring a practically constant current density along all of the immersed length of the wire. The electrolysis solution which is utilized for providing this protective layer has the following composition:

The value of the pH is adjusted by the addition of ammonia or hydrochloric acid in the electrolytic solution. This solution is employed at a constant temperature, the value remaining between substantially 55 C. and 70 C. The nickel-tin alloy is electro-deposited on the wire with a constant density current having a value which will be specified hereinafter.

The amount of nickel in the deposited alloy resins with the temperature of the electrolytic bath and with the density of the cathodic current. In the accompanying figure, curve represents the variation of the proportion of nickel (shown as ordinates) in the deposited nickel-tin alloy, as a function of the current density 1' (shown as abscissas, in ma./cm. for the above-mentioned electrolyte when the temperature of the bath is 50 C. Similarly, curves 2, 3, and 4 represent the variation of the proportion of nickel in the deposited nickel-tin alloy, as a function of the current density 1', for the above-mentioned electrolyte when the temperatures of the bath are respectively 55 C., 60 C. and 70 C. These four curves, which have been determined experimentally point-by-point, are provided by Way of example to show the relations of temperature and of current density involved at the time of the deposit of a nickel-tin alloy of given composition. From these curves can be determined the values of current density to utilize according to the temperature of the bath, in order to obtain the nickel-tin alloy possessing the desired property; i.e., a nickel-tin alloy in which the content of nickel is between 29% and 37%. For example, at a bath temperature of 60 C. this alloy is obtained by utilizing a current density greater than 10 ma./cm. On the other hand, at a bath temperature of 70 C. the current density for obtaining this alloy is greater than 34 maJcmP.

However, as has been indicated above, it is also necessary that the protective layer of electro'deposited nickeltin alloy have a thickness between 5,000 angstroms and a few microns, in order that the magnetic film is effective- 1y protected and that eddy currents are suppressed. For obtaining an alloy thickness satisfying this requirement, it is necessary that the electrolytic depositing of the alloy on the magnetic film take place during a precisely determined interval, which must be less as the value of the current density utilized is increased. Experiments have been conducted on coating this alloy onto a magnetic film of some hundreds of angstroms thickness deposited on a copper wire of 130 microns diameter, starting with the above-mentioned electrolyte. The object of these experiments was to determine the quantity of electricity required for obtaining a protective layer of thickness satisfying the requirement indicated above. It was determined that to obtain such thickness a quantity of electricity of between about 0.05 and 0.1 coulombs per centimeter of length of wire must be provided. In the example being considered, because the relative change in diameter of the wire produced by depositing the protective layer is negligible, the product obtained by multiplying the current density, expressed in milliamperes per square centimeter, by the time of electrolysis, expressed in seconds, must be between about 1250 and 2500 millicoulombs per square centimeter. Since the values of current density for obtaining the nickeltin alloy possessing the desired properties are known, it is simple to determine the duration of the electrolytic deposition. Thus, if the operation takes place at a temperature of 60 C., to obtain the nickel-tin alloy containing 29% to 37% of nickel, a current density greater than ma./cm. must be employed. If, for this temperature, a current density equal of 20 ma./cm. is selected, it is necessary that the duration of the alloy deposition operation be between about 62 seconds and 125 seconds. The accompanying figure shows that if the coating operation takes place under these conditions; i.e., at 60 C. and with a current density of 20 ma./cm. the nickel-tin alloy deposited on the magnetic film contains about 34.5% of nickel.

In the example described, wherein the magnetic film is deposited on a very long copper wire of 130 microns diameter, the electrolytic solution utilized for forming the protective coating is maintained preferably at a temperature of 65 C. The velocity of the wire through the tank holding this electrolyte is about 10 meters per hour. From the figure, the current density for a bath temperature of 65 C. to obtain a nickel-tin alloy having the desired composition must be greater than 20 ma./cm.. In the instant example, a preferred current density of 26 ma./cm. was selected, whereby the time required for each point of the wire to be immersed in the bath is between about 48 seconds and 96 seconds. In the example considered, a time of about 1 minute was selected. Therefore, considering the velocity of the wire lead to adopting, for the deposition of the protective coating, a tank providing for immersion of the Wire over a length of about centimeters. By operating under these conditions; i.e., at 65 C. and with a current density of 26 ma./cm. there is obtained on the wire a protective layer of nickel-tin alloy containing 36% of nickel, having a suitable thickness for assuring effective protection of the magnetic film, and suppressing eddy currents.

In the example which has been described the thickness of the protective layer can be varied, while maintaining the same tank and operating at the temperature of 65 C., by changing the current density. Nevertheless, the current density must be greater than ma./cm. in order to obtain a deposit of nickel-tin alloy of suitable composition. Because, in this example, each point of the wire is immersed for about 1 minute, the current density can be adjusted to a constant value between a minimum of: 1250/60=20 ma./cm. and a maximum of: 2500/60=40 ma./cm.

It has been mentioned above that the electrolysis process described for depositing the nickel-tin alloy provides for obtaining a coating uniformly distributed over all the surface of the magnetic film. Automatic regulation of the thickness of the deposited layer occurs because the resistance of the nickel-tin alloy is relatively high, whereby points on the magnetic film which become covered with even a very thin coating of the nickel-tin alloy present a much higher resistance to the electrolysis current than points on film not yet covered by the alloy.

In addition to the avantages indicated above, this protective coating is of very low cost. Furthermore, not only does this protective coating not alter the magnetic properties of the film on which it is deposited but further, it does not disturb the structure of the film when, after having been covered with its protective layer, the magnetic coated wire is introduced into an oven in order to be subjected to the well-known operation of annealing for stabilizing its magnetic properties. It has been verified, moreover, that this nickel-tin alloy which contains 29% to 37% of nickel, is virtually not attacked when it is subjected to the action of air, of oxygen, of water, of strong bases such as sodium or potassium hydroxide, or of strong acids such as sulfuric or nitric acid. Furthermore, this alloy particularly well resists the action of mercury, which thereby provides the capability of utilizing contacts of mercury of large area to provide for the passage of currents which are applied to the wire substrate of the magnetic film when, at the output of the annealing oven, the wire is introduced into an apparatus for measuring the magnetic properies of the film. For this purpose, reference can be made to the French patent application PV 6,917,172, filed May 27, 1969, and the corresponding US. patent application Ser. No. 39,032 filed May 20, 1970, by R. Girard et al., for Process for Treating Substrates of Thin Magnetic Films, both applications being assigned to the assignee of the instant invention. In such application there is described an experimental apparatus (FIG. 3 thereof), intended for studying the magnetic properties of a thin film deposited on a cylindrical wire substrate. The contacts 14, 15, 22 and 23 which appear on the drawing of this figure can be advantageously replaced by mercury contacts, which introduce virtually no frictional resistance on the wire and leave intact the coating of nickel-tin alloy on the magnetic film. This coating, despite its high electrical resistivity, offers only a negligible resistance to the passage of current because of the fact that it is so very thin.

Much that has been described in the foregoing and that is represented on the drawing is characteristic of the invention. It is evident that one skilled in the art is able to adduce all modifications of form and detail using his judgment, without departing from the scope of the invention.

What is claimed is:

1. A coated memory element comprising;

a conductive substrate in the form of a length of cylindrical wire,

a magnetic film supported on said substrate and consisting of about 18% iron with the remainder essentially nickel, and

a magnetically-protective layer adhered to said magnetic film and formed of an alloy consisting of between 29% and 37% nickel and with the remainder essentially tin, and having a thickness suflicient to preserve the pristine magnetic properties of said magnetic film, yet insufficient to produce eddy currents to any significant degee.

2. A coated memory element as set forth in claim 1 wherein said protective layer has a Vickers hardness between 400 and 700.

3. A process for producing a memory element having a conductive substrate with a thin magnetic film deposited thereon which includes the steps of:

providing a conductive substrate in the form of a cylindrical wire having a magnetic film consisting of about 18% iron with the remainder essentially nickel supported thereon,

and electrodepositing a magnetically-protective layer upon said magnetic film, said magnetically-protective 7 8 layer being an alloy consisting of between 29% and 3,077,285 2/ 1963 Budininkas 29-196.6 XR 37% nickel with the remainder essentially tin. 3,077,421 2/1963 Budininkas 29-1966 XR References Cited GERALD L. KAPLAN, Primary Examiner UNITED STATES PATENTS 5 US. Cl. X.R.

2,926,124 2/ 1960 Taylor et a1. 204-43 29-194, 196.3, 196.4, 196.6, 199; 204-27, 28, 40, 43;

3,268,424 8/1966 Brown et a1 204-40 XR 340-174 PW 

